How can I feel nothing for an ex?

I have to work with my ex. We have been broken up for 8 months now after dating for about a year. We used to be outside of work friends and hung out a lot. I never saw him as anything more than a friend. He, however, had a huge crush on me. So much so, that it had gotten to the point where he couldn't keep it to himself anymore (he said he kept thinking about me all the time). So he asked me out.

I resisted him a LOT at first. But then I fell for him. I felt like as soon as I really started falling for him...he pulled away. Its true that he temporarily got busy at work. But he didn't even talk to me about it! He just came over to my apartment on a weekday and ended it. After the breakup, I could tell he still had very strong feelings for me because he would tell me...but we never got back together.

I am very hurt how someone goes from caring so much about me to not giving a crap about me? He seems like I'm not even a tiny thought on his mind at all anymore. I did not do anything to deserve it. I know he was not in a stage in his life where he could commit...but I wasn't asking for a commitment! I only wanted to keep dating. But he just ended everything we had like it meant nothing! And I know he isn't dating anyone else, because we have a lot of mutual friends that would have told me.

I want to know how to not give a crap about an ex I have to see everyday? I want to feel absolutely nothing for him! Treat him like another random acquaintance. A part of my brain still misses him and thinks about him and always remembers "oh! this is your ex, you should try and impress him!" I just don't want to spend another single thought on him! Has anyone done this, and how?